U.S. Pat. No. 2,159,967, Engelmann, issued May 30, 1939, discloses carboxylic acids and their salts having an amine oxide substituent at the alpha-carbon atom. The compounds are generally described as being surfactants which can be used for or in admixture with soaps and soap substituents.
However, it has been found that the alpha-amine oxides have stability problems which can seriously affect their usefulness as detergent surfactants. It is believed that heavy-metal ions, such as copper, cobalt and particularly iron ions, form chelates with the alpha-amine oxides and catalyze their decomposition to relatively insoluble, non-surface active alpha-amino compounds. Trace amounts of such heavy metal ions (e.g., on the order of parts per million or less) normally present in detergent compositions can cause substantial decomposition of the alpha-amine oxides over a period of time.
The instability of the alpha-amine oxides is also partly due to the fact that structurally they are secondary amine oxides (i.e., the carbon atom next to the amine oxide substituent is attached to 2 other carbon atoms, instead of just one carbon atom as with primary amine oxides). As such, they decompose according to the Cope elimination reaction more readily than the primary amine oxides commonly used in the detergent industry (e.g., the alkyl dimethylamine oxides). However, since the alpha-beta unsaturated acids or salts formed by Cope elimination provide some detergency, the aforementioned metal-catalyzed decomposition represents the more serious stability problem.